1. Field of the Invention
This invention relates to a method for preparing high alpha-type silicon nitride powder.
2. Prior Art
There is expected an increasing demand for sintered silicon nitride as refractory structural material since it has improved high-temperature strength, thermal shock resistance and corrosion resistance. In particular, silicon nitride powder having an alpha-type crystalline phase content (to be referred to as .alpha.-conversion, hereinafter) of 90% by weight or higher is designated high alpha-type silicon nitride powder and known to exhibit extremely high material strength at elevated temperature when sintered. Prior art well-known industrial methods for the preparation of silicon nitride include (1) direct nitridation, (2) reducing nitridation, and (3) halogenated imidization. Most commonly used among others is the direct nitridation method which is cost effective.
As is well known in the art, the direct nitridation method involves exothermic reaction to generate heat as much as 176 kcal per mol as shown by the following chemical formula. EQU 3Si+2N.sub.2 .fwdarw.Si.sub.3 N.sub.4 +176 kcal
It is therefore desired to restrain vigorous reaction to achieve a reaction temperature distribution as uniform as possible. For a tunnel furnace used as a reactor, it was proposed to adjust the charge to a tray or to control the heating rate. Also Japanese Patent Application Kokai (JP-A) Nos. 97110/1986, 266305/1986 and 60410/1991 disclose the use of a rotary kiln and fluidized bed as the reactor for providing a uniform reaction temperature.
From another point of view, the direct nitriding reaction is a gas-solid system reaction between gaseous nitrogen and solid metallic silicon. It is said that the reaction is apparently determined by diffusion of nitrogen gas into metallic silicon or into the resulting silicon nitride. It would occur to those skilled in the art to increase the reaction temperature or reaction pressure in order to increase the diffusion rate. In the silicon nitride-forming reaction, however, more .beta.-type silicon nitride forms at higher temperature and the desired .alpha.-type silicon nitride is not obtained in a reasonable yield. As to the pressure, it is reported that pressure has little effect unless it is extremely high (see Surface, Vol. 24, No. 7, page 363, 1986). For commercial scale production, the use of a reactor capable of maintaining such an extremely high pressure is neither economical nor practical.
One well-known solution to this problem is by mixing the reaction gas with hydrogen gas.
Also proposed is a catalytic method which is relatively simple to produce high .alpha.-type silicon nitride powder. Exemplary catalysts heretofore known to be effective for this purpose include potassium, sodium and lithium compounds as disclosed in JP-A 128698/1975, magnesium oxide as disclosed in JP-A 48800/1976, metallic iron and iron compounds as disclosed in JP-A 15499/1979, alkaline earth metal halides as disclosed in JP-A 22000/1979, aluminum nitride as disclosed in JP-A 57499/1979, palladium oxide as disclosed in JP-A 58700/1979, calcium compounds as disclosed in JP-A 120298/1979, copper compounds as disclosed in JP-A 92906/1984, and vanadium compounds as disclosed in JP-A 256906/1986.
We found that among these catalysts, copper or copper compounds are by far most effective for producing high .alpha.-type silicon nitride powder. The method of the above-referred JP-A 92906/1984 requires that copper or copper compounds be added in amounts as large as 0.5 to 10% by weight based on the weight of metallic silicon, and it is rather difficult to purify the product or remove the copper catalyst. More particularly, conventional purification used in the art is typically acid treatment. When acid treatment was applied to a system loaded with a large amount of copper or copper compound, it was difficult to completely remove the copper or copper compound from the system. In this case, the resulting silicon nitride powder was molded and sintered into products which had strength below the desired level. Moreover, the addition of at most 10% by weight based on metallic silicon of a copper catalyst is undesirable from the point of view of economic production of silicon nitride.